Stabilization of myc proto-oncogene proteins during Friend murine erythroleukemia cell differentiation.

A rapid and dramatic decrease in c-myc mRNA has been associated with the differentiation of a variety of cell types and may be a critical step in the maturation process. In this study, we have simultaneously measured steady-state c-myc protein and c-myc mRNA levels in differentiating Friend murine erythroleukemia (MEL) cells. Northern blot analysis of poly(A+) RNA indicated a greater than 85% decrease in c-myc transcripts following a brief exposure to the inducing agents, dimethyl sulfoxide or hypoxanthine. The short half-life of the c-myc protein (approximately 30 min) suggests that its level should fall in a similar fashion. Surprisingly, immunoblots of total cell proteins and immunoprecipitations of 35S-labeled protein revealed that c-myc protein levels remain approximately constant. This unexpected finding was accounted for in part by an increase in the c-myc protein half-life to 75-85 min. Immunoprecipitation of [35S]methionine-labeled proteins also demonstrated that the undifferentiated MEL cell synthesizes equal amounts of two c-myc-related proteins, p59 and p61. In contrast, MEL cell populations in the late stages of differentiation express predominantly the higher molecular weight species. This latter observation represents the first report of a temporal shift in the relative abundance of the two c-myc isoforms during cell differentiation.